Proof of Work (PoW) is a process that requires miners to expend energy solving a sort of puzzle for each block added to the blockchain. After it’s complete, the “work” of solving new blocks can be easily verified by other miners.
PoW also serves as a protection mechanism against bad actors who might try to alter the blockchain’s transaction record or spam the network with meaningless transactions.
What’s the work?
Mining work comes from a process called hashing, wherein miners guess at a random encrypted number included in every block called a nonce. The nonce is needed for miners to win the lottery to make the next block. Miners make guesses as fast as possible to be the first to discover the number and claim the reward for their work.
PoW effectively creates a form of artificial cost in a usually costless digital world. Anyone wishing to amend or alter the blockchain’s transaction record must complete expensive work that is cheap to prove and confirm.
After a miner wins the guessing race, the answer (nonce) is broadcasted to other miners across the network to share that the work has been completed and the block is added to the network. In this way, the work is proven and easily confirmable by other miners.
With the cost of PoW comes the benefit of security.
Bitcoin users can trust that the network’s transaction record is valid because only values that miners have expended energy on can make it onto the ledger. Re-writing these transactions, moreover, requires overpowering a majority of the network, which is about the energy equivalent of a small country.